In heat exchangers such as air conditioners, refrigerators, etc., heat transfer tube is employed to convey a refrigerant which changes its phase between liquid and gas to create an exchange of heat relative to any fluid outside the tube. Continuous spiral grooves defined on the inner surface of the tube in, for example, a heat exchanger such as a room air conditioner promote the thermal conduction from the evaporation and condensation process in the tube. The continuous spiral grooves act to increase the heat transfer area, and the turbulent refrigerant elevates heat transfer rate. A cross grooved heat transfer tube is also available wherein two types of grooves set at different angles to each other with respect to the longitudinal axis are added. This intensifies fluid turbulence to improve the heat transfer properties.
Heat transfer tubes based on this cross grooved principle are described in, for example, Japanese Patent Application Laid-Open Nos. Sho 57-58092, Sho 60-29593, Hei 6-221788, Hei 8-42987, Hei 8-61878, Hei 8-178547, Hei 8-42978, and Sho 56-59194.
Among them, cross grooved heat transfer tube (hereinafter referred to as "the first inner surface-grooved heat transfer tube") as described in Japanese Patent Application Laid-Open No. Sho 57-58092 is constituted as having primary and secondary grooves rotating in opposite directions to each other defined on the inner wall surface thereof, with the secondary grooves shallower than the primary grooves. This means that a liquid film produced on the surfaces of protuberances (fins) forming on the primary grooves will drop to the bottom of the tube due to gravity all the while flowing over primary and secondary grooves by surface tension. Improvement in the heat transfer rate through condensation is intended.
A cross grooved heat transfer tube (hereinafter referred to as "a second inner surface-grooved heat transfer tube") described in Japanese Patent Application Laid-Open No. Sho 60-29593 is a tube wherein ribs (fins) are formed by defining the first grooves each of which has a predetermined angle with respect to the longitudinal axis of the tube, and on which the secondary grooves being shallower than the primary grooves are defined on the ribs at a predetermined angle with respect to the longitudinal axis going around towards the direction opposite to the primary grooves. Improvement in the heat transfer performance of single phase flow is intended on the basis of the above described constitution.
A cross grooved heat transfer tube (hereinafter referred to as "a third inner surface-grooved heat transfer tube") disclosed in Japanese Patent Application Laid-Open No. Hei 5-221788 is a tube wherein a plurality of fins substantially parallel to the longitudinal direction of the tube are provided on the inside wall of the tube so that primary grooves are constituted between each two fins and spiral notches are defined on the same fins at a predetermined angle with respect to the longitudinal axis of the tube. These notches constitute secondary grooves. This heat transfer tube is manufactured in a manner where a strip of copper or a copper alloy is rolled to form the fins. Notches are then defined by rolling and embossing the same with accompanying formation of burr. Finally the materials thus processed, are subjected to seam welding to obtain a tubular structure. In this heat transfer tube, improvement in heat transfer performance is intended by specifying the depth of the notches to be at least 40% of the height of the fins.
A cross grooved heat transfer tube (hereinafter referred to as "a fourth inner surface-grooved heat transfer tube") disclosed in Japanese Patent Application Laid-Open No. Hei 8-178574 is a tube wherein main grooves intersecting subsidiary grooves are provided on the inside of the tube, and three-dimensional projections involving burr in the front and rear thereof are formed on the fins constituting the main grooves.
A cross grooved heat transfer tube (hereinafter referred to as "a fifth inner surface-grooved heat transfer tube") disclosed in Japanese Patent Application Laid-Open No. Hei 8-42987 is a tube wherein fins are formed on the inside of the tube, and notches to interrupt the fins at predetermined pitches, respectively, are defined on the fins.
A cross grooved heat transfer tube (hereinafter referred to as "a sixth inner surface-grooved heat transfer tube") disclosed in Japanese Patent Application Laid-Open No. Hei 8-61878 is a tube wherein the depth of the notches has been increased to define grooves on the inner surface of the tube in the cross grooved heat transfer tube as disclosed in Japanese Patent Application Laid-Open No. Hei 8-42987.
A cross grooved heat transfer tube (hereinafter referred to as "a seventh inner surface-grooved heat transfer tube") disclosed in Japanese Patent Application Laid-Open No. Sho 56-59194 is a tube wherein fins involving notches defined thereon at a predetermined gap, are formed on the inside of the tube, and concave portions communicating with the inner space of the tube through fine openings are defined under the grooves defined between each two fins.
A heat exchanger utilized as in air conditioners, refrigerators or the like requires a condenser in which the fluid flowing through the interior of a tube will change from gas to liquid, and an evaporator in which the fluid will change from liquid to gas. Since condensers and evaporators are optimized to be in accordance with the environments to which they are applied, no sufficient performance is obtained in other environments. In this respect, the use of the heat transfer tube depends on the suitability of the condenser and evaporator to be applied.
In recent years, overcoming environmental problems such as global warming, the depletion of the ozone layer, acid-rain, and pollution of the oceans has become a significant challenge. One restriction is the use of CFCs as these deplete the ozone layer. 99.5% CFC-R22 (HCFC-22) which has until now been employed as a refrigerant for air conditioners will be also banned in 2020. The use of CFC-R22 has already been decreased. The selection of R407C for use in packaged air conditioners and R410A for use in room air conditioners is decisive.
The new refrigerants are mixed refrigerants of two or three types. R407C is a refrigerant prepared from three CFCs R32, R125, and R134a so as to achieve substantially the same physical properties as those in currently used R22 wherein respective refrigerants evaporate and condense at different temperatures to one another. The resulting R407C is referred to as a zeotropic mixed refrigerant. On the other hand, R410A is a refrigerant prepared by mixing R32 with R125 at a ratio of 1:1. The refrigerant exhibits a substantially azeotropic state, so there is no decrease in heat transfer performance, but its operating pressure is about 1.6 times higher than that of R22. In these circumstances, the heat transfer tube is used for both condensation and evaporation, so it requires a different constitution from that of conventional heat transfer tubes.
However, in accordance with the conditions outlined in the descriptions in the conventional first to seventh inner surface-grooved heat transfer tube patents, if the depth of the secondary grooves defined on the fins forming the primary grooves is not suitable, pressure loss in the refrigerant will increase with subsequent loss in performance of condensation and evaporation.
At the same time, an increase in pump power consumption to prevent boundary formation between the gas and liquid constituting the zeotropic mixed refrigerant is undesirable.
On top of this, in accordance with the third and fourth inner surface-grooved heat transfer tubes descriptions where the depth of the secondary grooves is specified to be at least 40% of a height of the fins, burr produced at the time of forming the secondary grooves leads to greater pressure loss.